This is a renewal application to further our investigation into the mechanism and significance of Bif-1-mediated Bax/Bak activation and mitochondrial morphogenesis in the context of tumorigenesis. Our studies have suggested that Bif-1 directly interacts with Bax in the mitochondrial outer membrane (MOM) and that both the physical and functional interactions between Bif-1 and Bax require the priming of monomer Bax by apoptotic stimuli that trigger Bax membrane recruitment, such as the BH3-only protein tBid. It appears that Bif-1 acts by promoting Bax homo-oligomerization rather than by facilitating Bax recruitment to the MOM. In addition, Bak activation is delayed as well in cells lacking Bif-1 during apoptosis. However, Bif-1 does not interact with Bak directly. Interestingly, our preliminary studies have demonstrated that Bif-1 selectively binds to the Bak inhibitor VDAC2. Bif-1, also known as endophilin B1, is a member of the endophilin protein family. Like other endophilins, Bif-1 is able to generate membrane curvature through its N-BAR domain, promoting Bax insertion into lipid bilayers. Moreover, Bif-1 has been shown to be required for fission of the MOM. Thus, we strongly suspect that Bif-1, in collaboration with BH3-only molecules, promotes Bax/Bak activation by altering the mitochondrial membrane shape. Moreover, Bif-1 and endophilin B2 (EndoB2) form stable heterodimers or tetramers in cells, suggesting that EndoB2 may also participate in the MOM remodeling associated with Bax/Bak activation during apoptosis. To further investigate the roles of endophilin B family proteins in Bax/Bak activation and mitochondrial morphogenesis, we propose the following Specific Aims: 1) To define the roles of Bif-1 and EndoB2 in the regulation of Bax/Bak activation, mitochondrial morphogenesis, and apoptosis; 2) To determine whether Bif-1 interaction with VDAC2 plays a role in the regulation of Bak oligomerization, mitochondrial morphology, and apoptosis; 3) To determine the roles of Bif-1 and EndoB2 in apoptosis, mitochondrial morphogenesis, and androgen-independent prostate cancer progression in mouse models under both physiological and pathological settings.